DESCRIPTION (Applicant's abstract): It has been long known that testosterone (T) can facilitate both male-typical and female- typical reproductive behaviors. However, the mechanism by which T affects both types of behavior is not yet fully understood. The objective of the proposed study is to further investigate these mechanisms at three different levels: 1) circulating levels of T; 2) conversion of T in the brain into the behaviorally-relevant metabolites estrogen (E) and dihydrotestosterone (DHT); 3) sensitivity of the brain to T and the metabolites. To test if the relationship between these mechanisms and specific reproductive behaviors extends to other species, a second species will be included in the study of T conversion. It is predicted that factors relevant to masculine behaviors should be enhanced in males, and factors relevant to female behaviors should be enhanced in females. Furthermore, a sex dimorphism in the factors should be enhanced in species whose reproductive behaviors are also sexually dimorphic. The first experiment will determine if a sex difference exists in T levels in the blood of males and females. The second experiment will measure the activity of the enzymes that convert T into E and DHT in the brain of males and females. Finally, sex differences in sensitivity of the brain to T will be determined by examining the distribution of T-receptors, as well as the receptors for the T metabolites, E and DHT.